1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic type or electrostatic recording type, and more particularly, to a technique for correcting tone characteristics.
2. Description of the Related Art
Recently, color multi-function peripherals and color printers have been increasingly installed at offices and copy shops. With such color image forming apparatuses, quality of output images tends to become unstable due to long-term use and environmental variation. To deal with this, it is desirable to form a test pattern on a recording medium and correct tone characteristics of the image forming apparatus by reading the recording medium.
Incidentally, there are color image forming apparatuses which, by holding multiple image processing modes, select and apply an image processing mode according to the content of their output. Each image processing mode is represented, for example, by a γ lookup table (γLUT) for correcting gamma characteristics. However, a contribution rate to characteristic variation of an image forming apparatus varies with the image processing mode. Therefore, tone correction must be performed for each image processing mode. That is, it is desirable to generate and read a test pattern for each image processing mode and thereby create an optimal γLUT for each image processing mode.
Conventionally, most such full-color, multi-function peripherals are expensive and large in size. Recently, however, inexpensive, small apparatuses have started to become popular due to technological advances. In small office/home office (SOHO) and personal uses, in particular, many small, full-color, multi-function peripherals specialized in output of small-size images have come to be used widely.
Even in the case of small, inexpensive, full-color, multi-function peripherals, achieving high image quality and high stability is becoming essential. However, in performing tone correction similar to conventional tone correction for characteristic variation of an image forming apparatus, it has come apparent that there are problems unique to small apparatuses. For example, since the output paper size of small apparatuses is smaller than the output paper size of large apparatuses, it is necessary to reduce the tone pattern size or reduce the number of tone patterns.
Also, the large apparatus forms test patterns for multiple image processing modes on different pieces of output paper. However, it is required of small apparatuses for SOHO or personal use that tone correction can be performed with minimal labor on the part of the user. That is, there is a desire to fit test patterns for different image processing modes on a single piece of output paper. To do so, it is desirable to reduce the size or number of tone patches that make up the test patterns.
However, reduction in the number of tone patches has the disadvantage of degrading the accuracy of the created γLUT. In particular, there is a danger of considerably degrading tone correction accuracy in a low density region and high density region. This is because these regions are influenced significantly by variation in tone characteristics caused by long-term use and environment. Therefore, it is not desirable to reduce the number of tone patches.
While it is desirable to reduce the size of tone patches without reducing the number of tone patches, reduction in the size of tone patches has the disadvantage of reducing reading accuracy of an image reading apparatus which reads the tone patches. This is because when the area of tone patches is reduced, the reading accuracy becomes susceptible to influence from the surface (so-called ground) of the output paper around periphery of the tone patches. Thus, to reduce the size of tone patches, a technique for reducing the influence of the ground is required.